A catalyst component for the polymarization of .alpha.-olefins, comprising magnesium or a metal oxide and magnesium, titanium, a halogen and an electron-donating compound is known. This catalyst of the metal oxide support type has jointly good polymerization properties such as high activity and high stereoregularity and excellent grain properties whereby polymers are obtained having narrow grain size distributions and uniform grain forms.
When the grain strength of a catalyst component is low, a polymer formed by polymerization is broken to give a fine powder. As a typical method for preventing this phenomenon, there is a so-called previous polymerization method comprising previously contacting the catalyst component with an olefin to take the thus formed polymer in the catalyst and thereby increasing the strength of the catalyst component. It has been proposed to improve the grain strength and increase the stereoregularity of the poly .alpha.-olefin by adding an electron-donating compound such as silane compounds during the previous polymerization.
However, the addition of an electron-donating compound during the previous polymerization ordinarily has influences such as lowering of the catalyst activity and deterioration of the properties during storage of the catalyst. As the silane compound to be added during the previous polymerization, there are often used compounds having aromatic groups from the standpoint of the properties, but depending on the object of using the polymer, the silane compounds having aromatic groups are harmful in some cases.
The inventors have made various studies for the purpose of increasing the strength of catalyst grains, improving the stereoregularity of the resulting polymer, maintaining the high activity of the catalyst and preventing the catalyst from deterioration of the property thereof during storage and consequently, have found that when using a dimethoxy silane-containing compound having a volume of 170 to 500.ANG..sup.3, in particular, 230 to 500.ANG..sup.3 and an electron density of oxygen in the methoxy group, ranging from 0.685 to 0.800 A. U., in particular, 0.690 to 0.800 A. U., as a silane compound to be added during previous polymerization, poly .alpha.-olefins can be obtained with properties, at least, comparable to those obtained when using organosilicone compounds having aromatic groups, and the object of the present invention can thus be achieved.